(2) Field of the Invention
The present invention relates to a plasma assisted chemical vapor deposition (CVD) process for the preparation of an adherent polycrystalline diamond thin film on a substrate, preferably silicon or glass, using relatively low temperatures (less than 600.degree. C.). In particular, the process relates to particular conditions for producing the adherent diamond thin film.
(2) Description of Related Art
Diamond has an exceptionally wide optical transmission window, is resistant to chemical attack at ordinary temperatures, and is expected to be highly resistant to abrasion and erosion (Harris, D. C., Proc. SPIE, 2286, pp. 218-228 (1994)). Consequently it is of interest as a coating material for other optical materials which are less resistant to hostile environments. The most common optical material is glass; however, diamond film deposition on glass can be problematic. Adhesive diamond films deposited by chemical vapor deposition (CVD) have been reported on quartz, which has a low thermal expansion coefficient (Ong, T. P., et al., Appl. Phys. Lett., 55 2063-2065 (1989); Pickrell, D. J., et al., J. Mater. Res. 6 1264-1276 (1991); Muranaka, Y., et al., J. Vac. Sci. Technol. A, 9 76-84 (1991); and Joseph, A., et al., 2nd International Conference on the Applications of Diamond Films and Related Materials, Eds. M. Yoshikawa, et al., 429-432, MYU, Tokyo (1993)). For glasses with lower softening point temperatures and larger thermal expansion coefficients than pure silica, such as borosilicate glasses, adhesion of CVD diamond films has been reported to be a problem (Nariman, K. E., et al., Chem. Mater. 3 391-394 (1991)).
Diamond is of interest for electronic packaging because of its combination of thermal, electrical and mechanical properties. Diamond is already being used as a substrate material onto which heat producing electronic components are attached (Harris, D. C., Proc. SPIE 2286 218-228 (1994); and Ong, T. P., et al., Appl. Phys. Lett. 55 2063-2065 (1989)). In such cases, the high thermal conductivity of diamond combines with its high electrical resistivity and low dielectric constant to provide high-performance heat sinks for microwave integrated circuits, power device modules, and multichip modules (Pickrell, D. J., et al., J. Mater. Res. 6 1264-1276 (1991); and Muranaka, Y., et al., J. Vac. Sci. Technol. A. 9 76-84 (1991)). However, diamond is also under consideration as an over-coating layer on microelectronic components. One potential advantage of diamond films on integrated circuits is as a superior passivation layer. In previous work, for example, hot-filament chemical vapor deposition (CVD) was used to deposit a 2 .mu.m thick diamond film on a silicon wafer containing circuitry including linear amplifiers and resistor elements used for thermal transfer printing devices (Joseph A., et al., 2nd International Conference on the Applications of Diamond Films and Related Materials, Eds., M. Yoshikawa, et al., 429-432, MYU, Tokyo (1993)). It was noted that the resulting devices exhibited superior lifetime in that the diamond coated circuits did not exhibit failure due to abrasion of the resistor elements by the paper.
However, the diamond deposition temperature of 850.degree. C. for this example is too high for conventional microelectronics which use aluminum metallization. overlay diamond films have also been proposed for improved thermal management of local hot spots in which, for example, diamond would be used as the dielectric material to fill the gap between source-gate and gate-drain regions of high electron mobility transistors (Nariman, K. E., et al., Chem. Mater. 3 391-394 (1991)). In order to implement such a device, low temperature deposition of diamond is required.
The prior art in chemical vapor deposition of polycrystalline diamond on various substrates is extensive. U.S. Pat. No. 4,925,701 to Jansen et al describes a process wherein diamond particles are used for seeding a surface to be coated to produce an adherent film on the surface as tested by the adhesive tape method. The films are particularly useful for protecting electronic circuitry on silicon wafers (IC chips) from heat and mechanical damage. Relatively high temperatures above 650.degree. C. were used by Jansen et al which can affect the circuitry on the IC chips. Patents which describe related processes are: U.S. Pat. No. 5,311,103 to Asmussen et al; U.S. Pat. No. 5,302,231 to Bovenkerk et al; U.S. Pat. No. 5,298,286 to Yang et al; U.S. Pat. No. 5,286,524 to Slutz et al; U.S. Pat. No. 5,270,077 to Knemeyer et al; U.S. Pat. No. 5,260,106 to Kawarada et al; U.S. Pat. No. 5,243,170 to Maruvama et al; U.S. Pat. No. 5,242,711 to DeNatale et al; U.S. Pat. No. 5,240,749 to Chow; U.S. Pat. No. 5,200,231 to Bachmann et al; U.S. Pat. No. 5,230,931 to Yamazaki et al; U.S. Pat. No. 5,204,144 to Cann et al; U.S. Pat. No. 5,188,862 to Itatani et al; U.S. Pat. No. 5,185,179 to Yamazaki et al; U.S. Pat. No. 5,183,685 to Yamazaki; U.S. Pat. No. 5,180,571 to Hosova et al; U.S. Pat. No. 5,145,711 to Yamazaki et al; and U.S. Pat. No. 5,028,451 to Ito et al.
It has been found that the films have relatively poor adhesion to a glass containing substrate either as a result of stresses developed between the film and the substrate in thermal cycling or because the film does not bond well to this substrate. The optical transmission properties of the prior art films are relatively difficult to control. There is a need for improvements in plasma assisted CVD of diamond on silicon containing substrates.